For use in refrigerators and refrigerated showcases, studies are recently under way on the replacement of chlorofluorocarbon refrigerants by hydrocarbon refrigerants which are less likely affect the depletion of the ozone layer or global warming. Since hydrocarbon refrigerants are inflammable, leakage of the refrigerant must be diminished to the greatest possible extent.
Heat exchangers adapted to prevent leaks of the refrigerant are known which are fabricated by arranging a plurality of fin groups at a spacing, each of the fin groups comprising parallel plate fins each having two pipe inserting holes, inserting two straight outer pipes through the respective holes of all the plate fins, forcing a pipe enlarging tool through the outer pipes to enlarge the pipes and fixedly fit the plate fins around the outer pipes, inserting two straight pipe portions of an inner pipe in the form of a hairpin and comprising a pipe having no weld seam into the respective outer pipes, and bending the outer pipes and the straight pipe portions of the inner pipe at portions between the adjacent fin groups to form the pipe assembly in a zigzag shape in its entirety (see the publication of JP-A No. 2001-124485).
Since the inner pipe has no weld seam, the fluid flowing through the inner pipe, i.e., the refrigerant when the conventional heat exchanger is used as the evaporator of a refrigeration device, is prevented from leaking.
Further with the conventional heat exchanger, the inner pipe is not enlarged by the pipe enlarging tool, so that inner fins can be formed on the inner peripheral surface thereof integrally therewith, hence an increased heat transfer area to result in an improved heat exchange efficiency.
However, the conventional heat exchanger has the problem of requiring a high material cost and having great weight in its entirety since the outer pipes and the inner pipe must be used. The heat exchanger has another problem in that the insertion of the straight pipe portions of the inner pipe into the outer pipes requires cumbersome work. Moreover, it is impossible to greatly reduce the difference between the outside diameter of the inner pipe straight portions and the inside diameter of the outer pipes so as to render the inner pipe straight portions insertable into the outer pipes with an improved work efficiency. This entails the problem of impaired intimate contact between the pipe portion and the outer pipe to result in a lower heat transfer efficiency and a lower heat exchange efficiency.
With the conventional heat exchanger described, an adhesive or compound of high heat transfer properties is provided between the inner pipe straight portion and the outer pipe so as to ensure improved heat transfer therebetween. The heat exchanger therefore has the problem that the adhesive or compound is cumbersome to apply.
Accordingly, to overcome these problems, it is thought useful to provide a heat exchanger by arranging a plurality of fin groups at a spacing, each of the fin groups comprising parallel plate fins each having two tube inserting holes, inserting two straight tube portions of a hairpin tube comprising a tube having no weld seam through the respective holes of all the plate fins, forcing a pressure fluid into the hairpin tube to enlarge the tube, fixedly fit the plate fins around the straight tube portions of the hairpin tube and form a heat exchanger finned tube, and bending the hairpin tube straight portions of the finned tube at portions between the adjacent fin groups to form the finned tube in a zigzag shape in its entirety.
However, this heat exchanger has the problem that the hairpin tube ruptures at a finless portion thereof which has no fins when the tube is enlarged with the pressure fluid.
An object of the present invention is to overcome the above problems and to provided a heat exchanger finned tube which is adapted to diminish the leakage of refrigerant and to exhibit the desired refrigeration performance and which can be prevented from rupturing during fabrication, a heat exchanger, an apparatus for fabricating the heat exchanger finned tube and a process for fabricating the heat exchanger finned tube.